Rename rsc/rsc-types.ts to rsc/index.ts to fix issues importing ai/rsc

[briann]

Potential fix for https://github.com/vercel/ai/issues/1028.

I verified this locally (yarn add ai@/path/to/this/branch) and I'm able to import from ai/rsc. However I'm not sure if this breaks something else that was intended by naming the type export as rsc-types.ts (if so, feel free to decline this PR).

[shuding]

It's weird and I don't understand why renaming the file fixes it...